FIG. 1 shows a conventionally used spark plug 9 for an internal combustion engine. For example, the spark plug 9 is used as a means for igniting an air-fuel mixture introduced into a combustion chamber of an internal combustion engine such as of a passenger car.
The spark plug 9 includes a center electrode 94 and a ground electrode 95. The ground electrode 95 has an end fixed to a housing 92, while being bent to bring the other end to a position facing the center electrode 94.
In the ground electrode 95, a projection portion 96 is arranged, being projected toward a spark discharge gap 911. The projection portion 96 has an opposing face 960 that faces the center electrode 94. As shown in FIG. 2 by (A) and (B), a discharge is caused in the spark discharge gap 911 and the air-fuel mixture is ignited by the discharge. A reference E in the figure indicates a discharge spark formed by the discharge, a reference F indicates a flow of the air-fuel mixture and a reference I indicates a flame (see Patent Document 1).
Patent Document 2 discloses a spark plug that includes a ground electrode without having the projection portion 96.